The present invention relates to heat sinks for integrated circuits, particularly to copper-diamond heat sinks, and more particularly to a copper-diamond composite material such as that having high thermal conductivity and to a process for fabricating the composite material.
Diamonds and composites comprised of diamond particles embedded in a metal matrix have been used for various applications due to the hardness and heat conductivity of diamonds. Both diamonds and diamond/metal composites have been used extensively in industrial applications, such as various types of tools. Due to the cost of diamonds, various methods of forming diamond/metal composites and processes for forming tools of various shapes from such composites have been developed. This prior effort is exemplified by U.S. Pat. No. 2,382,666 issued Aug. 14, 1945 to I. A. Rohrig et al.; and U.S. Pat. No. 5,096,465 issued Mar. 17, 1992 to S. Chen.
With the advent of integrated circuits, and the need for adequate heat sinks therefor, researchers utilized substrates of diamonds and metals, such as copper, as a means for dissipating heat from the integrated circuits. For example, a Type II natural diamond has a thermal conductivity of 20 W/cmK compared to 4 W/cmK for copper. The high thermal conductivity of Type II diamond makes it the most attractive material for heat sink applications. Unfortunately, Type II diamonds are expensive and only available in relatively small sizes.
In efforts to resolve the cost and obtain sufficient heat dissipation, small sized diamonds (heat spreaders) were mounted in larger metal substrates (heat sinks), such as copper. Thus, performance was improved by mounting circuits on heat spreaders that increase the thermal footprint of the circuit resulting in more efficient cooling. These prior efforts are exemplified by U.S. Pat. No. 4,425,195 issued Jan. 10, 1984 to N. A. Papnicolaou; and U.S. Pat. No. 4,800,002 issued Jan. 24, 1989 to J. A. M. Peters.
Synthetic diamond films fabricated by a chemical vapor deposition (CVD) process (14 W/cmK) are almost comparable in heat conductivity to natural diamonds. However, the cost for these synthetic diamonds is still prohibitively expensive for mass produced electronic applications. Copper with a thermal conductivity of 4 W/cmK is a very attractive heat sink material. However, its high thermal expansion makes it incompatible with semiconductor materials and established integrated circuit fabrication processes. A similar problem exists with diamond because of its very low coefficient of thermal expansion. The brittle nature of diamond presents still another serious technical problem to its use as a thermal conducting substrate in large integrated circuit designs.
Diamond/metal composite materials are attractive for integrated circuit heat sinks because of the low-cost and the compatibility of thermal expansion with semiconductor materials (i.e. Ga, As, Si). The thermal conductivity and thermal expansion of a composite material are approximately equal to the volumetric average of the properties of the components in the composite. A composite material with 60% by volume of Type II diamond particles and 40% copper would have a thermal conductivity approximately equal to that of CVD diamond: EQU 20 W/cmK (0.6)+4 W/cmK (0.4).about.13.6 W/cmK
The thermal expansion of this composite material would be a similar fractional average of the thermal expansion of each component and similar to that of semiconductor materials.
Research efforts were also directed to the development of effective integrated circuit heat sinks using diamond/metal composites. They primarily involved hot-pressing of a diamond-metal powder compact as the fabrication technique. These composite development efforts are exemplified by U.S. Pat. No. 3,912,500 issued Oct. 14, 1975 to L. F. Vereschagin; U.S. Pat. No. 5,008,737 issued Apr. 16, 1991 to R. D. Burnham et al.; U.S. Pat. No. 5,120,495 issued Jun. 9, 1992 to E. C. Supan et al.; and U.S. Pat. No. 5,130,771 issued Jul. 14, 1992 to R. D. Burnham et al. While these efforts advanced this field of technology, modern high density integrated circuits are still limited in power, speed of operation, packing density, and lifetime by thermal considerations, and primarily the availability of suitable heat-sink material. While the prior hot-pressing techniques reduced the costs of fabricating the composite heat sinks, the thermal conductivity was low compared to the ideal value calculated for a diamond/metal composite. Thus, there has been a need for a low cost composite material which can effectively function as a heat sink or heat spreader for high density integrated circuits.
This need in the high density integrated circuit art is solved by the present invention which constitutes a new type of composite material with a thermal conductivity comparable to the calculated value based on the volumetric concentration of diamond and metal in the composite. This new material consists of up to 75% by volume diamond particles in a thermally conducting metal matrix (i.e. copper-silver). This matrix or composite material can be fabricated in relatively large sizes by a new process which involves infiltration rather than hot pressing. The use of diamond powder coated with layers of different metals allows the intimate bonding to the metal matrix material required for optimum thermal conductivity.